Hello all ok so I made a power supply out of my 500w psu itall worked fine but I wanted to make it neater so I desolderd all of the wiresfrom the psu board and left Ground/ +5/+12/-12 and soldered a permanent wirebetween the ps-on and com so it would turn on every time I flip the switch, Theproblem is when I flip the switch the psu turns on for lets say 3 seconds thenit dies then if left for a short while I flip the switch and same 3 secondsonly, So any ideas would be great as I’m hopeing I haven’t killed the psu L

Hi XsamX,
Is that with or without an Amiga connected? Maybe the power supply shuts down when there's no load. I don't know which rail needs to get some load (5 or 12 volt), but you may try to add some load to see if the powersupply stays on.

Hello all ok so I made a power supply out of my 500w psu itall worked fine but I wanted to make it neater so I desolderd all of the wiresfrom the psu board and left Ground/ +5/+12/-12 and soldered a permanent wirebetween the ps-on and com so it would turn on every time I flip the switch, Theproblem is when I flip the switch the psu turns on for lets say 3 seconds thenit dies then if left for a short while I flip the switch and same 3 secondsonly, So any ideas would be great as I’m hopeing I haven’t killed the psu L

I think you need a momentary switch to the "ps on" signal, just like on a normal PC.

Hello all ok so I made a power supply out of my 500w psu itall worked fine but I wanted to make it neater so I desolderd all of the wiresfrom the psu board and left Ground/ +5/+12/-12 and soldered a permanent wirebetween the ps-on and com so it would turn on every time I flip the switch, Theproblem is when I flip the switch the psu turns on for lets say 3 seconds thenit dies then if left for a short while I flip the switch and same 3 secondsonly, So any ideas would be great as I’m hopeing I haven’t killed the psu L

A 500W PSU will have remote sense on +5V and +3.3V. In the original wiring loom there should have been smaller wires in parallel with the normal power wires. Their purpose is to make a Kelvin sense connection, which allows the PSU controller chip to adjust the output voltage to keep operating within the ATX supply limits under all load conditions. If you disconnect the sense wires, the PSU will shut down as a safety feature, if it did not the output could easily rise to 50V, thus destroying a lot of electronics.

What are the power ratings printed on the label?

You may need a 15 ohm, 10W or better resistor on the +12V supply, your answer to the above question will help.

I don't believe a momentary switch is required, I have modified several ATX PSU's to be permanently on with this method without problems.

If you do resort to using a resistor to add additional load, I would secure it to the case of the PSU so that the case will act as a large heatsink. Avoid using plastic cable ties or anything else which may melt.

However, finding an older, lower wattage unit would be a far better option in my opinion.

I don't believe a momentary switch is required, I have modified several ATX PSU's to be permanently on with this method without problems.

If you do resort to using a resistor to add additional load, I would secure it to the case of the PSU so that the case will act as a large heatsink. Avoid using plastic cable ties or anything else which may melt.

However, finding an older, lower wattage unit would be a far better option in my opinion.

Sounds good to me, What Resistor Do you Recommend and where do I solder it to

If you use a Switch to connect "PS ON" (usually the green cable) to ground (black cable) on the ATX 20 or 24 pin connector then it must be a toggle on and toggle off switch type that continuously connects "PS ON" to ground when in the toggled on position.

Using a ATX case's "momentary" or a push to connect power switch to connect "PS ON" to ground on the ATX 20 or 24 pin connector will only power the ATX power supply on while there is a connection between the "PS ON" and ground. That is why the ATX powers off when you release the button.

The ATX case's "momentary" power switch is supposed to be connected to special circuitry on the ATX motherboard that is powered by the ATX power supply's "5V standby" power line (usually a purple/violet cable). This special motherboard circuit takes the "PS ON" connection low (to nearly 0 volts) to turn the ATX power supply on and to a higher voltage to turn the ATX power supply off. The special circuit also allows an alarm in the BIOS to turn the power on and running software on the ATX motherboard to turn the power off.

Some ATX power supplies also need a reasonable load on the ATX power supply's main +5V supply line to maintain power regulation and to stay powered on. Avoid placing the "stay on" 5V load on the +5V standby power line as that will fail to keep the power on.

Note the -5V supply line may be missing on the ATX main 20/24 pin connector on Modern ATX power supplies. The -12v supply may also be current limited to 0.3A or 300mA, and may disappear completely on later ATX versions.

As said, it's nothing to do with a momentary switch - pulling (and holding) the _PS_ON signal low is the ATX standard method of activating the supply. The momentary action just feeds into the motherboard's power control circuitry that actually does the switching on and off of the PSU.

Also bear in mind that, for many ATX PSUs these days, it's the +12V rail that's the "primary" output and the one which needs to have a minimum load. This is due to modern PCs taking most of their power from 12V instead, in particular modern GPUs. As a general rule (but of course there are exceptions), the highest power output from a PSU like this is typically the main output and the one which needs to be loaded. So check which rail outputs the most watts, and then see about loading that. Sometimes a couple of old hard drives might be enough, but the best bet is a resistor.

A decent ATX power supply will shut itself down to protect the computer when it can't stabilise its output within spec. This could be because of the sense wire being disconnected as Stedy suggested, or simply not enough load. This is probably better than a lesser PSU that doesn't have the safety feature built in, which could happily supply too high a voltage to your Amiga.